In a five star hotel, I ordered for a cup of coffee. When I was about to sip, I found a fly in the coffee. I complained. The manager apologized, took back the cup and brought another fresh hot one. But I realized that he bought the same coffee. I sued the hotel in a consumer court and won compensation. The question is not how I won the case. How could I recognize that the manager has bought the same coffee?
The cup has an unique IPv6 address?
Here is one possibility
You had already added the sugar before observing the fly. This time when you added the sugar again, it was extra sweet. You produced that cup of coffee with video recording that you added sugar just once still coffee contains double sugar.
You put salt in it before you sent it back. That's a flavor that just doesn't belong in your coffee.
You already added the milk/sugar?
The fly was still there?
The coffee had been in the cup long enough that the outside of the cup had had time to heat up?
You had not told the manager how you take your coffee (managers of 5 star hotels are rarely at ground level taking orders), so there is no way the manager knew how to make a new cup.
The customer in question is female, and there was lipstick on the cup.
the cup was chipped and so recognizable.
I don't expect the waiter to be stupid enough to bring a coffee already mixed with milk again. There might have been some color on the cup from your lips (assuming you are a woman) but again he should have seen that.
You could have told from the temperature which decreases when the cup is carried around. Maybe there was no steam in the air, no smell. But you said he brought a "fresh hot one" which confused me. I also thought about the surface of the coffee that changes from a gleaming one to a dull, black one after some time. Plus, the fly or the object that picked the fly could have had influence on that.
You could still taste the fly in the cup.
The manager came back too fast. In a five star hotel, one can expect good, fresh made coffee - and such coffee needs >= 5 minutes in preparation and letting it steep.
This is because good coffee loses too much flavour when not grinded literally just seconds before brewing.
So if the manager came back in under five minutes, he could not bring a new cup of coffee.
"But I realized that he bought the same coffee."
"How could I recognize that the manager has bought the same coffee?"
The receipt was left on your table. In plain text you saw: Medium Fly Coffee $2.99
The ear of the cup was warm from being in the microwave?
If the hotel had a system like Starbucks where you get your name on the coffee cup, you may have realized that it was the same cup.
"The manager apologized, took back the cup and brought another fresh hot one."
He brought back another fresh, hot cup. The coffee in the cup was the same coffee, and still had the fly in it.
the same batch and variety of coffee, just a different cup of it. The fly may not have been in the cup, but actually lay among the original source of the coffee.
The cup was hot, but the coffee was lukewarm
The fly was was a fly of a different sort...
You did what everybody would do in these cases. You put the cup in a plastic bag and sent it to the lab for fingerprints. And the fly for autopsy.
If you find your own fingerprints, that means it is the same cup.